1. Field of the Invention
The present disclosure relates generally to ion implantation and more particularly to techniques for measuring ion beam current.
2. Description of Related Art
Ion implanters are commonly used in the production of semiconductor devices, flat panels, solar cells and so on. An ion source is used to generate a charged ion beam, which is then analyzed by a mass analyzer to remove ions with undesired charge-to-mass ratios and then directed toward a workpiece, such as a semiconductor wafer or a glass plate, held by a holder. One or more devices may be located between the mass analyzer and the holder for adjusting the ion beam by applying an electric field, a magnetic field and/or other approach. These devices, such as collimator and acceleration/deceleration electrodes, are usually viewed as a portion of the “beam optics”. To monitor the ion beam current, which is significantly related to the dosage in the implanted workpiece, a Faraday cup is commonly used to receive and measure the ion beam. Usually, the Faraday cup is a deep structure that extends into a chamber wall and has an opening that faces a workpiece position where a workpiece held by the holder is located. By moving one or more of the holder and the ion beam, the ion beam may be directed toward the Faraday cup and then be measured correspondingly.
FIG. 1 is a representative schematic diagram of the configuration of the conventional Faraday cup. An ion beam 12 from the mass analyzer (not shown) is directed from the left side of a chamber 11 to a Faraday cup 13, which is located on the right side of the chamber 11 and is a deep structure that extends into a chamber wall of the chamber 11. The Faraday cup 13 has an opening facing an inner space of the chamber 11, such as a workpiece position where a workpiece to be implanted is located. Clearly, when the ion beam 12 is projected into the Faraday cup 13, the charges of these ions in the ion beam 12 will be measured by a current meter 14 electrically coupled with the Faraday cup 13. In addition, a separate set of magnets may be located close to the Faraday cup 13 for adequately suppressing secondary electrons, incoming electrons and low energy ions, so that the current measured by the current meter 14 is accurately equal to the current of the charges delivered by the ion beam 12.
However, there are some practical problems. First, the ion beam 12 from the mass analyzer, or through the beam optics, may be misaligned. Then, the ion beam 12 may be not totally projected into the Faraday cup 13. Second, owing to space charge effects, the expansion of the ion beam 12 is inevitable. The beam expansion is more serious for a low energy ion beam, because the slower ion velocity results in longer travel time from the mass analyzer to the Faraday cup 13. Then, especially for the low energy ion beam, the ion beam 12 tends to be tall and wide and then a significant percentage of the total beam current is at the edges. Hence, when the ion beam is not totally received by the Faraday cup 13, the accuracy of the ion beam measurement is further degraded. Third, with the popularity of larger-size workpieces, such as 12-inch wafers, there is a tendency toward taller ion beams. Clearly, when the size of the entrance of the Faraday cup 13 is limited, the taller beams present a risk of projecting part of the ion beam 12 outside the Faraday cup 13.
A popular approach to achieve accurate dose control is to use a profiler to measure the ion beam 12. Hence, by using the acquired beam current distribution of the ion beam 12, the current measured by the Faraday cup 13 may be corrected. However, the hardware cost and the operation of the profiler will increase the total cost and decrease the throughput. Another popular approach is directly increasing the size of the entrance of the Faraday cup 13 for increasing the cross-section area capable of receiving the ion beam. However, much hardware exists on the chamber wall of the chamber 11, for example, the gas pipeline connected to the vacuum pump for pumping, the power line for powering the beam optics or the devices for moving the holder, and the window for moving the workpiece in and out of the chamber 11. Hence, the size of the conventional Faraday cup, especially the size of the opening of the conventional Faraday cup, cannot be arbitrarily enlarged.
A recent approach is U.S. patent application Ser. No. 12/841,833 to Peter M. Kopalidis, filed Jul. 22, 2010, incorporated herein by reference. In this approach, a planar Faraday cup is disposed on the inner surface of the chamber wall and optionally around the conventional Faraday cup which is a deep structure as described above, and a magnet is positioned close to the planar Faraday cup so that at least some kinds of undesired charged particles are suppressed by the magnetic field generated by the magnet. In this way, the effective area to receive the ion beam is increased, and the inaccuracy induced by the undesired charged particles (such as incoming electrons, secondary electrons and slow ions) can be improved. However, this approach is still not perfect. For example, the existence of the magnet unavoidably increases the hardware cost and the size of the chamber.
Accordingly, a novel and efficient approach for the above issue is desired.